Sonatype release from travis

[labianchin]

Release is currently manual and it does not run in travis.

To release make sure to:

• Have access to Sonatype, follow this: https://foss.spotify.net/docs/manual/03-releasing-software/03-releasing-by-language/java.html
• Have a GPG key available, follow this: http://central.sonatype.org/pages/working-with-pgp-signatures.html
• Have sbt-pgp configured, follow this: http://www.scala-sbt.org/sbt-pgp/usage.html
  To release the open source library run the following (it takes 78 min):

git clone git@github.com:spotify/dbeam.git dbeam_release1
cd $_
git config user.email $USER@users.noreply.github.com
git checkout master && git branch --set-upstream-to=origin/master master
time sbt '+publishLocalSigned' 'release cross with-defaults'

We should investigate having the release performed by travis.

A few references:

https://alexn.org/blog/2017/08/16/automatic-releases-sbt-travis.html
https://github.com/scalacenter/sbt-release-early/wiki/How-to-release-in-Travis-(CI)
https://github.com/making/travis-ci-maven-deploy-skelton
https://github.com/spotify/spydra/blob/master/.travis.yml#L11

[labianchin]

Closed by #43